Color is a psychological phenomenon based on the interaction of the spectrum of light (distribution of light energy versus wavelength) with light receptors in the eye, which have specific sensitivities for specific spectral wavelength bands or regions (hereinafter “spectral components”) of light. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra.
In the physical sense, a color is the ratio of intensities measured in different spectral components of the electromagnetic spectrum. In the physiological sense, in humans, the different spectral components are defined by the spectral sensitivity curves of three different types of receptors in the eye (i.e., the so-called red, blue, and green cones). The human brain processes and combines signals from the red, blue and green cones to create a composite impression or image of a scene. All colors in the scene are perceived as combinations of the red, blue and green cone signals. The range or gamut of colors that are perceived by humans is represented, for example, by the CIE 1931 chromaticity diagram (FIG. 5).
Man-made color image sensors (e.g., color film, or digital cameras using CCD or CMOS sensors) also sense light intensities in a finite number of distinct spectral components. Various types of color image sensors differ in how they separate and measure the distinct spectral components. For example, a color film may have a stack of three different emulsion layers that are exposed by red, green, and blue components of light, respectively. A digital camera may use an array of layered sensors so that every pixel, like a color film, contains a stack of sensors sensitive to individual colors (e.g., sensor available under the Foveon trademark). More commonly, digital cameras use a spatial color filter array (e.g., a Bayer filter) positioned on top of a CMOS or CCD sensor to capture different spectral components of light in corresponding nominal pixel types.
The man-made color image sensors sense and collect intensity data for each distinct spectral component received from a scene. The data for each spectral component is monochrome, i.e., it includes only intensity information, but no color information. To create an approximate color image or rendition of a scene, the different intensity data for the distinct spectral components are processed, encoded to certain colors and intensities, and combined.
The color image or rendition is an approximation, at least in part, because the collected intensity data does not include full or complete information on the spectral distribution of light received from the scene. The intensity data on the spectral distribution of light received from the scene is limited by, for example, the finite number of spectral components sensed, the shape and the widths of the sensor spectral sensitivities for each spectral component, and overlap of the spectral sensitivities for the different spectral components. FIG. 3 shows the red, blue and green spectral responses of an exemplary CMOS sensor with a RGB color filter. Consideration is now being given to ways of increasing or supplementing the color image data with more information on the spectral distribution of light received from the scene.